The present invention relates to a system and a method for establishing the instantaneous speed in a safe manner of an object which is travelling along a known trajectory, in particular a vehicle such as a train.
Patent application WO 02/03094 filed on 29 Jun. 2001 describes a system for locating an object in a safe manner, such as a train, based on the transmissions of signals carried out by a group of GNSS (Global Navigation Satellite System) satellites.
In the context of railways, safe location is understood in the sense of the standard Cenelec 50129. It relates to providing a location interval with an extremely low probability (from 10−09 to 10−12) that the train can be outside that interval.
With regard to establishing the instantaneous speed, prior systems conventionally based on electromechanical devices have the disadvantage of being extremely costly. In order to improve safety, they use a combination of sensors which detect axle rotation and which have to withstand accelerations greater then 90 g, single-axle accelerometers and on-board radar systems.
Therefore, the object of the invention is to provide a system for establishing the instantaneous speed having the level of safety required by railways at a cost far lower than existing systems.
Therefore, the invention relates to a system for establishing the instantaneous speed in a safe manner of an object which is travelling along a known trajectory, in particular a vehicle such as a train, the system comprising:                means for approximately locating the object and direction of its movement, and        means for receiving at least one signal transmitted at a predetermined frequency, known to the object, from at least one satellite and        means for calculating and analyzing the signal comprising:        means for measuring the frequency shift of the signal received relative to the predetermined frequency of the signal transmitted, which means are connected to        means for establishing the vectorial difference of the speeds of the object and the satellite in the satellite/object direction by calculating the Doppler effect which has generated the frequency shift and        means for calculating the instantaneous speed in the direction of movement of the object based on the vectorial difference established previously. The measurements of the frequency shift and the associated calculations are repeated independently on signals from at least two different satellites and the calculation and analysis means further comprise means for combining the results obtained independently in order to obtain an instantaneous speed with the desired level of safety.        
Other features of the invention are:                the means for calculating and analyzing the signal for the object are adapted to establish the frequency shift owing to the Doppler effect of two signals, each being from two separate satellites, the means for establishing the instantaneous speed of the object being adapted to establish that speed based on the frequency shift of the two signals;        it uses the signals from four satellites, the combination means using the instantaneous speeds obtained by the calculation means based on six pairs of satellites, each satellite belonging to at least one pair;        the means for calculating and analyzing the signal further comprise stable time reference means, the means for establishing the instantaneous speed of the object being adapted to establish that speed based on the frequency shift owing to the Doppler effect of a single signal from a single satellite;        it uses the signals from three satellites; and        it comprises means for receiving the signal which are located at two separate points of that object and which are capable of simultaneously receiving the signal at those two points, the receiving means being connected to means for calculating the path travelled by the signal which are capable of establishing that the signal has reached the two points of the receiving means via different paths and removing, for a predetermined period of time, the satellite which transmitted that signal, from the list of satellites for which the speed calculations previously defined are authorised.        
The invention further relates to a method for establishing the instantaneous speed
of an object which is travelling along a known trajectory, which method comprises the steps of:                establishing the approximate location and movement direction of the object,        receiving at least one signal transmitted at a predetermined frequency, known to the object, from at least one satellite,        measuring the frequency of the signal received by the object,        measuring the frequency shift of the signal received relative to the predetermined frequency of the signal transmitted,        establishing the vectorial difference of the speeds of the object and the satellite in the satellite/object direction by calculating the Doppler effect which has generated the frequency shift, and        calculating the instantaneous speed of the object in the movement direction of the object based on the instantaneous speed established previously.        